The present invention relates to a powder intended for developing latent images, and to a method of producing the powder.
A particular, although not exclusive, application of this powder is in the field of printing machines, termed non-impact or strikeless transfer printing machines, in which characters are printed without relying for this purpose on the impact of raised printing characters against a recipient sheet of paper, the machines being of the magnetic type in the present case.
Printing machines of this type have an image carrying member which is usually formed by a rotary drum of an endless belt on the surface of which it is possible, by magnetic means, to form sensitized zones, also termed latent images, which correspond to the characters to be printed. These latent images are then developed, that is to say rendered visible, by means of a powdered developing pigment which, when deposited on the image carrying member, is only attracted by its sensitized zones. The powdered pigment is applied by means of an applicator arrangement of a known kind in front of which the image carrying member passes. Thereafter, the particles of pigment which have been deposited on the latent images are transferred to a carrier sheet, such as a sheet of paper, for example, to which they are then permanently affixed.
The powdered developing pigments which are used in magnetic printing machines have to meet far more stringent requirements than those which are used in other non-impact printing machines. Such other machines are either (1) electro-optical machines which have an image carrying member on which surface latent images are formed by electrostatic means, the said images being developed by means of a developing pigment which is then transferred to a sheet of ordinary paper or, (2) electrostatic machines in which the latent images, instead of being formed on an image carrying member, are generated directly on a sheet of special paper which has been treated beforehand to enable it to record such latent images. Powdered developing pigments intended for magnetic non-impact printing machines in fact need to be capable not only of adhering to the magnetized zones of the image carrying member but also of being easily transferred, in their entirety, to the recipient sheet of paper with which they are subsequently brought into contact.
The pigments need to be capable of melting freely at a relatively low temperature, so that the temperature of the arrangement for affixing them to the paper can be set at a level such that there is no risk of the paper charring or catching fire. Nevertheless, the pigments must not have too low a melting point as otherwise they will be liable to soften, which will make them sticky and cause them to adhere more strongly to the image carrying member, thereby preventing their being transferred to the paper in their entirety. Furthermore, when they have not yet been melted, the pigments should not cake or become charged with static electricity, to prevent their sticking to the parts of the image carrying member which have not been magnetized. It is also necessary that the pigments not produce a disagreeable odor or dangerous vapors when they are melted. Furthermore, they must be capable of mixing properly with the coloring agents which must often be added to increase the contrast between the background shade of the paper and the color of the images formed on the paper.
It is essential that the powdered pigments have a fine particle size so that images of a good quality can be produced on the paper. In addition, the pigments, when melted, should not spread out over the paper or diffuse into it in order to prevent blurring. Nor should the pigments be affected by moisture. Finally, their properties should remain virtually unaltered with time.
The powdered developing pigments which have been used hitherto in magnetic non-impact printing machines do have some of the characteristics mentioned above. This is true for example of the powdered developing pigment which was mentioned incidentally in the French patent application published as No. 2305764, which pigment is formed from ferromagnetic materials and insulating resins. However, there is no developing pigment which possesses all of the above-named ties simultaneously.
The present invention overcomes the deficiencies of the prior art and provides a powder which, being intended for the development of magnetic images, has all of the properties mentioned above.